An image sensor converts an optical image focused on the sensor into electrical signals. Typically, the image sensor comprises an array of light sensitive elements, each element producing a signal corresponding to the light intensity incident on that element. The signals may then be used, for example, to provide information about the optical image.
Some image sensors integrate a light sensitive device with signal processing circuits in each pixel of the array. Such image sensors are generally classified into either analog types or digital types. The analog type is generally suitable for light integration, but is far from robust against noise and therefore has less precision. The digital type is robust and more suitable to communication. However, it requires an Analog-to-Digital (A/D) converter and the capability to perform arithmetic operations, such as addition and multiplication, which are area consuming.
One common type of image sensor used in many consumer applications, such as digital cameras and camcorders, is a charge-coupled device (CCD). Some of the appeal of CCDs derives from their ability to produce an image and maintain their resolution when the illumination intensity is low. However, integrated circuits comprising CCD image sensors have a number of drawbacks including a relatively low yield, relatively high power consumption and a high cost of production, which is due to the specialized processing involved.
A much cheaper alternative to CCD image sensors are Complementary Metal Oxide Semiconductor (CMOS) image sensors that have a light sensitive element such as a photodiode, phototransistor or other suitable device, where the conductivity of the light sensitive device corresponds to incident light intensity on the element. A variable analog signal can therefore be generated by the light sensitive element. Light sensitive elements may be formed in a two dimensional core array, which is addressable both by row and by column. Once a row of elements has been addressed, the analog signals from each of the sensitive devices in the row are coupled to the respective columns in the array. An A/D converter may then be used to convert the analog signals on the columns to digital signals so as to provide only digital signals at the output of the image sensor that is typically formed in an integrated circuit.
U.S. Pat. No. 5,012,344, assigned to K.K. Toshiba, discloses a solid-state image pickup device that endeavors to address the complex and expensive manufacturing processes and insufficient signal-to-noise-ratios (SNRs) encountered with conventional image sensors. The image sensor disclosed employs a plurality of charge/voltage conversion elements in correspondence with a plurality of photosensitive elements. A control pulse generation circuit controls the integration of the signal charges, which ultimately leads to an improved SNR and simplified manufacturing of the image sensor. However, this image sensor still exhibits relatively high power consumption.
The introduction of semiconductor memory at the pixel level allows imaging data to be stored locally and accessed in a manner similar to a standard Dynamic Random Access Memory (DRAM). However, pixel level A/D conversion has a disadvantage of operation voltage reduction that accompanies deep sub-micron processes. This may directly affect the signal quality and thus deteriorate the SNR. In addition, if the area of a photodiode decreases then the signal capacity in the photodiode decreases causing degradation of the SNR.
Further to the above, A/D converters implemented at the pixel level often use very simple and robust circuits and operate in parallel. However, such an implementation requires a large pixel size, has high output data due to over-sampling, has poor low light performance and has inherent fixed pattern noise due to the analog components required. An example of a CMOS image sensor with pixel level A/D conversion is disclosed in U.S. Pat. No. 5,461,425, assigned to Stanford University. In the image sensor disclosed, analog signals generated by phototransistors are converted to a serial bit stream by an A/D converter connected at the output of each phototransistor and formed in the immediate area of each phototransistor within an array core. The provision of a separate digital stream output for each pixel element of the array core endeavors to minimize parasitic effects such as parasitic capacitances, resistances and inductances, to which large area pixel arrays are susceptible. This sensor uses Sigma-Delta modulation and Nyquist samped data rates. Although this sensor is useful, there is a need for a two-dimensional image sensor that possess improved signal processing characteristics and reduces signal deterioration effects due to inherent D/A conversion that is required in these types of image sensors.
In this specification, including the claims, the terms ‘comprises’, ‘comprising’ or similar terms are intended to mean a non-exclusive inclusion, such that a method or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.